CN115916727A

CN115916727A - Silicon nitride sintered compact, rolling element and bearing using same

Info

Publication number: CN115916727A
Application number: CN202180050691.4A
Authority: CN
Inventors: 中村文耶; 早川康武
Original assignee: NTN Corp
Current assignee: NTN Corp
Priority date: 2020-09-03
Filing date: 2021-09-01
Publication date: 2023-04-04
Also published as: JP7307255B2; US20230303454A1; EP4209472A1; WO2022050290A1; JP2023121828A; JP2023009067A; JP2023126276A; JP2023130419A; JP2023121827A

Abstract

The invention provides a silicon nitride sintered body which has excellent mechanical properties, particularly excellent fracture toughness, and has excellent product life when processed into a product, a rolling element using the same, and a bearing. The silicon nitride sintered body contains a rare earth element and an aluminum element, the rare earth element is contained in an amount of 6 to 13 wt% in terms of oxide, the aluminum element is contained in an amount of 2 to 13 wt% in terms of oxide, the surface layer portion being a region within 2mm from the surface of the silicon nitride sintered body, and the proportion of the total cross-sectional area of the inclusions (I) to the total cross-sectional area of the surface layer portion is 0.05% or more.

Description

Silicon nitride sintered compact, rolling element and bearing using same

Technical Field

The present invention relates to a silicon nitride sintered body, a rolling element and a bearing using the same.

Background

Due to silicon nitride (Si) ₃ N ₄ ) Since the sintered body has excellent mechanical properties, thermal conductivity, and electrical insulation, it is being applied to bearing members, engine components, tool materials, heat dissipation substrate materials, and the like. It is known to produce a silicon nitride sintered body using a silicon nitride powder as a starting material. Since silicon nitride powder is difficult to sinter, a sintering aid is used together with the silicon nitride powder to produce a densified silicon nitride sintered body. As such a sintering aid, oxides of rare earth elements, alumina, magnesia, silica, and the like are generally cited, and it has been also studied to use a material containing a transition metal element as a sintering aid in order to improve the mechanical properties of a silicon nitride sintered body (for example, patent documents 1 and 2).

Since silicon nitride powder is expensive, when silicon nitride powder is used to produce a silicon nitride sintered body, the price of the silicon nitride sintered body tends to increase. Therefore, a method for producing a silicon nitride sintered body by using a silicon powder (metal silicon powder) which is less expensive than a silicon nitride powder as a starting material and subjecting the starting material to reaction sintering has been attracting attention (for example, patent documents 3 to 5). As such a production method, a method called the PS-RBSN (Post-Sintering of Reaction Bonded Silicon-Nitride) method is known. The PS-RBSN method comprises: a1 st step of nitriding a compact formed by molding a silicon powder by performing a heat treatment at a temperature of, for example, about 1100 to 1450 ℃ in an atmosphere containing nitrogen gas; a 2 nd step of densifying the nitride obtained in the 1 st step by heat treatment at a temperature in the vicinity of 1600 to 1950 ℃.

Documents of the prior art

Patent document

Patent document 1: japanese patent laid-open publication No. 2013-234120

Patent document 2: international publication No. 2015/099148

Patent document 3: japanese laid-open patent publication No. 2004-149328

Patent document 4: japanese patent laid-open No. 2008-247716

Patent document 5: japanese laid-open patent publication No. 2013-49595

Disclosure of Invention

Problems to be solved by the invention

When a silicon nitride sintered body is produced by the PS-RBSN method, if the silicon powder is not sufficiently nitrided, silicon remains in the silicon nitride sintered body. Since residual silicon may cause a reduction in mechanical properties of the silicon nitride sintered body, the silicon nitride sintered body produced by the PS-RBSN method may have inferior mechanical properties compared to the silicon nitride sintered body produced using silicon nitride powder as a starting material. Further, it has been found that when the silicon nitride sintered compact is processed into a product such as a rolling element, the product life may be short.

The purpose of the present invention is to provide a silicon nitride sintered body which has good mechanical properties, particularly good fracture toughness, and has a good product life when processed into a product, and a rolling element and a bearing using the silicon nitride sintered body.

Means for solving the problems

The silicon nitride sintered body of the present invention is a silicon nitride sintered body containing a rare earth element and an aluminum element, and is characterized in that the content of the rare earth element is 6 wt% or more and 13 wt% or less in terms of oxide with respect to the total weight of the silicon nitride sintered body, and the content of the aluminum element is 6 wt% or more and 13 wt% or less in terms of oxide with respect to the total weight of the silicon nitride sintered body.

Characterized in that the rare earth element contains 1 or more selected from Y, ce, nd and Eu. Further, it is characterized in that the rare earth element contains Ce.

The silicon nitride sintered body is characterized in that an inclusion (I) is present in a surface layer portion which is a region within 2mm from the surface, and the proportion of the total cross-sectional area of the inclusion (I) to the total cross-sectional area of the surface layer portion is 0.05% or more.

Characterized in that the inclusion (I) contains an inclusion (It) containing a transition metal element. Further, the inclusion (It) is a silicide of a transition metal element.

Characterized in that the transition metal element contains 1 or more selected from Ti, cr and Mn. Further, the transition metal element contains Cr.

Characterized in that the maximum diameter of the inclusions (I) is 50 [ mu ] m or less.

The silicon nitride sintered body is characterized in that the silicon nitride sintered body has pores in a surface layer portion which is a region within 2mm from the surface, and the maximum diameter of the pores is 50 [ mu ] m or less.

The rolling element of the present invention is characterized by using the silicon nitride sintered body of the present invention.

The bearing of the present invention is characterized by using the rolling elements of the present invention.

The bearing is mounted on a drive unit including a plurality of rotary blades and a motor for rotating the rotary blades, an electric vertical lift flying by the rotation of the rotary blades, and a bearing for supporting a rotary shaft in the drive unit.

ADVANTAGEOUS EFFECTS OF INVENTION

According to the present invention, a silicon nitride sintered body having excellent fracture toughness and exhibiting excellent product life when processed into a product, and a rolling element and a bearing using the silicon nitride sintered body can be provided.

Drawings

Fig. 1 is a longitudinal sectional view showing an example of a bearing according to the present invention.

Fig. 2 is a perspective view of an electric vertical lift device having the bearing according to the present invention mounted thereon.

Fig. 3 is a partial sectional view of a motor in a driving portion of the electric vertical lift.

Detailed Description

Embodiments of the present invention will be described below.

(silicon nitride sintered body)

The silicon nitride sintered body of the present embodiment contains a rare earth element and an aluminum element. In the silicon nitride sintered body, the content of the rare earth element is 6 wt% or more and 13 wt% or less in terms of oxide with respect to the total weight of the silicon nitride sintered body, and the content of the aluminum element is 6 wt% or more and 13 wt% or less in terms of oxide with respect to the total weight of the silicon nitride sintered body.

Examples of the rare earth element include yttrium (Y), lanthanum (La), cerium (Ce), samarium (Sm), neodymium (Nd), dysprosium (Dy), europium (Eu), and erbium (Er). Among them, yttrium (Y), cerium (Ce), neodymium (Nd), and europium (Eu) are preferable. In particular, cerium (Ce) is more preferably contained from the viewpoint of further promoting nitridation and improving production efficiency.

The content of the rare earth element is 6 wt% or more, preferably 6.5 wt% or more, and may be 7 wt% or more. The content of the rare earth element is 13 wt% or less, may be 12 wt% or less, and may be 11 wt% or less. By setting the content of the rare earth element within the above range, a silicon nitride sintered body having good fracture toughness and good product life when processed into a product can be easily obtained.

The rare earth element is derived from, for example, a sintering aid (usually an oxide of a rare earth element) containing a rare earth element used in the production of a silicon nitride sintered body. When the content of the rare earth element in the silicon nitride sintered body is within the above range, the nitriding reaction of the silicon powder (metal silicon powder) as a raw material can be promoted and the subsequent sintering can be promoted in the case of producing the silicon nitride sintered body by the PS-RBSN method. The PS-RBSN method is a 2-stage sintering method including a silicon nitriding step and a subsequent sintering step. The content of the rare earth element can be adjusted by the addition amount of the sintering aid containing the rare earth element (e.g., oxide of the rare earth element) added to the raw material.

The content of the aluminum element is 6 wt% or more, preferably 6.5 wt% or more, and may be 7 wt% or more. The content of the aluminum element is 13 wt% or less, may be 12 wt% or less, and may be 11 wt% or less. The content of the aluminum element (in terms of oxide) may be within ± 5 wt%, or within ± 2 wt%, or within ± 1 wt%, of the content of the rare earth element (in terms of oxide), or the same as the content of the rare earth element. When the content of the aluminum element is in the above range, a silicon nitride sintered body having good fracture toughness and a good product life when processed into a product can be easily obtained.

The aluminum element is derived from, for example, a sintering aid (usually alumina) containing aluminum used in the production of a silicon nitride sintered body. When the content of the aluminum element in the silicon nitride sintered body is within the above range, sintering can be promoted in the case of producing the silicon nitride sintered body by the PS-RBSN method. The content of the aluminum element can be adjusted by the amount of the sintering aid (for example, alumina) containing the aluminum element added to the raw material.

The above contents of the rare earth element and the aluminum element may be determined using a fluorescence X-ray analysis device (XRF), an energy dispersive X-ray analysis (BDX), or a high frequency Inductively Coupled Plasma (ICP) luminescence analysis device. Specifically, the content of the rare earth element and aluminum element in the silicon nitride sintered body can be determined by the above-mentioned analyzer and converted into an oxide (RE) of the rare earth element (RE) ₂ O ₃ Or REO ₂ ) And alumina (Al) ₂ O ₃ ). The elements constituting the other components of the silicon nitride sintered body may be analyzed by the above-mentioned analyzer to calculate the total weight of the silicon nitride sintered body and determine the above-mentioned contents of the rare earth element and the aluminum element. The raw material powder for producing the silicon nitride sintered body contains silicon (metal silicon powder) which becomes Si by nitriding ₃ N ₄ In the case of (3), si in the silicon nitride sintered body ₃ N ₄ The weight of (a) is 1.67 times the weight of silicon. Therefore, if the weight change at the time of silicon nitriding is taken into consideration, the contents of the oxide of the rare earth element and alumina can be calculated from the composition of the raw material powder.

The silicon nitride sintered body of the present embodiment preferably has inclusions (I) in a surface layer portion which is a region within 2mm from the surface. The inclusion (I) contains components other than silicon nitride, and examples thereof include an inclusion (It) containing a transition metal element, an inclusion (Is) containing a non-nitrided silicon element, and the like. The inclusion (It) is preferably a silicide of a transition metal element. The inclusion (Is) Is, for example, an aggregate of non-nitrided silicon element. The inclusion (I) preferably contains the inclusion (It), and preferably contains no inclusion (Is) or a small proportion thereof. The inclusions are inclusions existing in the entire surface layer part, which is a region within 2mm from the surface of the silicon nitride sintered body.

The inclusion (It) is derived from, for example, a sintering aid (usually, an oxide of a transition metal element) containing a transition metal element used in the production of a silicon nitride sintered body, for example, a silicide of a transition metal element formed in the production of a silicon nitride sintered body. When a silicon nitride sintered body is produced by the PS-RBSN method, the use of a sintering aid containing a transition metal element can promote the nitriding reaction of the silicon powder and the growth of needle-like crystals of silicon nitride. Therefore, the heat treatment time required for nitriding silicon can be suppressed, and the energy efficiency in producing the silicon nitride sintered body can be improved.

On the other hand, when the raw material for producing the silicon nitride sintered body contains silicon nitride powder, the silicon nitride powder and chromium oxide (Cr) are mixed ₂ O ₃ ) When the sintering aid containing a transition metal element (oxide of a transition metal element) is mixed, the sintering aid oxidizes the silicon nitride powder, and the composition of the raw material varies, and thus satisfactory sintering may not be performed. In contrast, when a silicon nitride sintered body is produced by the PS-RBSN method, since silicon powder is mainly used as a raw material and the content of silicon nitride powder contained in the raw material can be reduced, such a problem as described above is less likely to occur, and a dense silicon nitride sintered body can be obtained.

The inclusion (Is) may be formed when the silicon powder (metal silicon powder) as a raw material Is insufficiently nitrided in the case of producing a silicon nitride sintered body by the PS-RBSN method. If a large-diameter inclusion (Is) Is present in the surface layer portion or the proportion of the inclusion (Is) increases, the mechanical properties such as fracture toughness of the silicon nitride sintered body tend to decrease, and the product life when processed into a product tends to decrease. The inclusion (Is) present in the surface layer of the silicon nitride sintered body Is preferably small, and more preferably absent.

The transition metal element is not particularly limited as long as it is contained between group 3 and group 11 of the IUPAC periodic table. The transition metal element is preferably 1 or more selected from Ti, cr and Mn, and more preferably Cr. By containing Cr as a transition metal element, the fracture toughness of the silicon nitride sintered body can be further improved.

The content of the transition metal element in the silicon nitride sintered body is preferably 0.1 wt% or more, more preferably 0.3 wt% or more, and may be 0.5 wt% or more, usually 5 wt% or less, may be 3 wt% or less, more preferably 2 wt% or less, and may be 1 wt% or less in terms of oxide, relative to the total weight of the silicon nitride sintered body. The above content of the transition metal element can be determined by the same method as the method for determining the content of the rare earth element and the aluminum element.

The maximum diameter of the inclusion (I) present in the surface layer portion of the silicon nitride sintered body is not particularly limited. Specifically, the maximum diameter of the inclusions (I) is 50 μm or less, preferably 40 μm or less, preferably 30 μm or less, preferably 25 μm or less, and usually 0.5 μm or more. The maximum diameter of the inclusions (I) in the surface layer portion means the diameter of the inclusion (I) having the largest diameter among the inclusions (I) existing in the surface layer portion. When the maximum diameter of the inclusion (I) is within the above range, the inclusion (I) is easily suppressed from becoming a fracture source, and thus a silicon nitride sintered body having good fracture toughness is easily obtained. Further, since the maximum diameter of the inclusion (I) is within the above range, it is easy to suppress the occurrence of defects caused by the inclusion threshing from the silicon nitride sintered body, and therefore, in the case of processing the silicon nitride sintered body into a product such as a rolling element of a bearing, a good product life can be easily obtained. The maximum diameter of the inclusion (I) can be adjusted, for example, according to the degree of nitriding of the silicon powder as the raw material, the amount and/or particle diameter of the sintering aid containing a transition metal element added to the raw material, and the type of the transition metal element.

In the cross section of the silicon nitride sintered body, the ratio of the total cross-sectional area of the inclusions (I) to the total cross-sectional area of the surface layer portion ([ total cross-sectional area of inclusions (I)/total cross-sectional area of surface layer portion ] × 100) is preferably 0.05% or more, and may be 0.1% or more, and may be 0.15% or more, and may be 0.3% or more, and may be 0.6% or more. The above ratio is usually 7.0% or less, and may be 3.0% or less, 2.0% or less, or 1.5% or less. The above-mentioned proportion of the inclusion (I) is a proportion of the total cross-sectional area of all the inclusions existing in the surface layer portion to the total cross-sectional area of the surface layer portion. When the ratio is within the above range, a silicon nitride sintered body having good fracture toughness and a good product life when processed into a product can be easily obtained. In addition, if the above ratio is too large, the inclusions are continuously threshed, which tends to adversely affect the results of the bearing life test. The above-mentioned proportion of the inclusion (I) can be adjusted, for example, according to the degree of nitriding of the silicon powder as the raw material, the addition amount and/or particle diameter of the sintering aid containing a transition metal element added to the raw material, and the type of the transition metal element.

The silicon nitride sintered body of the present embodiment preferably has pores in a surface layer portion which is a region within 2mm from the surface. Further, the maximum diameter of the pores is preferably 50 μm or less in the cross section of the silicon nitride sintered body. The maximum diameter of the pores may be 40 μm or less, 30 μm or less, 25 μm or less, or no pores may be present. When the maximum diameter of the hole is within the above range, a good product life can be easily obtained when the silicon nitride sintered body is processed into a product such as a rolling element of a bearing. The voids in the surface layer portion are voids present in the surface layer portion, which is a region of the silicon nitride sintered body within 2mm from the surface, and are voids present throughout the surface layer portion. The maximum diameter of the pores in the surface layer portion means the diameter of the pores having the largest diameter among the pores in the surface layer portion. In the case of producing a silicon nitride sintered body by, for example, the PS-RBSN method, the maximum diameter of the pores can be adjusted by adjusting the content of silicon nitride used as a raw material and/or the addition amount of a sintering aid.

The maximum diameter of the inclusion (I), the above-mentioned ratio of the inclusion (I), and the maximum diameter of the void are values measured for the inclusion (I) or the void existing in the entire surface layer portion in the cross section of the test piece manufactured by the method described in the examples described later. The maximum diameter of the inclusions (I), the above-mentioned ratio of the inclusions (I), and the maximum diameter of the pores can be calculated by the methods described in the examples described later.

As described later, the silicon nitride sintered body according to the present embodiment is mainly manufactured by the PS-RBSN method. The silicon nitride sintered body produced by the PS-RBSN method has a smaller shrinkage than a sintered body using silicon nitride powder as a raw material because the relative density of the green compact is increased by nitriding once. The shrinkage ratio is calculated from the following equation. The "size" in the following formula is the size of the corresponding portion in the green compact and the silicon nitride sintered body. For example, when both are spherical, each diameter or the like can be used.

Shrinkage ratio [% ] [ { (size of green compact) - (size of silicon nitride sintered compact) }/size of green compact ]. Times.100

The shrinkage ratio of the silicon nitride sintered body of the present embodiment is not particularly limited, but is preferably 15% or less, may be 14% or less, and may be 13% or less from the viewpoint of dimensional accuracy of the sintered body and the like. The shrinkage rate may be, for example, 7% or more, 8% or more, or 10% or more.

In addition, the silicon nitride sintered body of the present embodiment contains the rare earth element in an oxide equivalent of 6 wt% to 13 wt% based on the total weight of the silicon nitride sintered body, and contains the aluminum element in an oxide equivalent of 6 wt% to 13 wt% based on the total weight of the silicon nitride sintered body, and in the production, for example, by using a considerable amount of the sintering aid containing the rare earth element and the sintering aid containing aluminum, the nitriding reaction can be sufficiently performed even when silicon powder is used as a raw material. As a result, good fracture toughness was obtained. The fracture toughness (according to JIS R1607) is, for example, 3MPa · m ^1/2 Above, preferably 4MPa · m ^1/2 More preferably 5MPa · m or more ^1/2 The above. Further, the fracture toughness is, for example, 8MPa · m ^1/2 The following.

A particularly preferable embodiment of the silicon nitride sintered body of the present embodiment is a silicon nitride sintered body containing a rare earth element and an aluminum element, and further, the silicon nitride sintered body has an inclusion (I) and a void in a surface layer portion which is a region within 2mm from a surface, a content of the rare earth element is 6 wt% or more and 13 wt% or less in terms of oxide with respect to a total weight of the silicon nitride sintered body, a content of the aluminum element is 6 wt% or more and 13 wt% or less in terms of oxide with respect to a total weight of the silicon nitride sintered body, a maximum diameter of the inclusion (I) present in the surface layer portion is 50 μm or less, a ratio of a total cross-sectional area of the inclusion (I) to a total cross-sectional area of the surface layer portion in a cross-section of the silicon nitride sintered body is 0.1% or more, and a maximum diameter of the void present in the surface layer portion is 50 μm or less. In this embodiment, the elements, the numerical ranges, and the like can be combined as appropriate.

The shape of the silicon nitride sintered body of the present embodiment is not particularly limited, and can be appropriately selected depending on the application, for example, a spherical shape, a cylindrical shape, a conical frustum shape, a rectangular parallelepiped shape, and is preferably a spherical shape. The size of the silicon nitride sintered body is not particularly limited, and for example, the diameter may be 0.5 to 10cm in the case of a spherical shape, the diameter of the bottom surface may be 0.5 to 15cm in the case of a cylindrical shape, and the height may be 3 to 20cm.

The silicon nitride sintered body is preferably produced by a PS-RBSN method (2-stage sintering method). Specifically, the following

methods

1 and 2 can be used for the production.

(method 1)

In the PS-RBSN method, granulation is often performed in order to improve the flowability of the powder. The 1 st method is a method for producing a silicon nitride sintered body containing a rare earth element and an aluminum element, and includes, for example: a granulation step of obtaining granulated powder using a raw material powder containing a silicon powder and a sintering aid; a molding step of molding the obtained granulated powder into a green compact; a degreasing process; and a sintering step of sintering the degreased green compact. After the sintering step, the silicon nitride sintered body may be polished or the like as necessary.

In the granulation step, the raw material powder and the binder component are mixed with water and/or an organic solvent (e.g., ethanol) to form a slurry, and the slurry is spray-granulated and dried by spray drying or the like to obtain a granulated powder. An organic binder or the like is used as a binder component, and is added to the whole raw material powder by, for example, 1 to 10 wt%.

In the next molding step, the granulated powder is molded into a predetermined shape to obtain a green compact. In the degreasing step, the obtained green compact is heated at a temperature of 700 to 1000 ℃ in a nitrogen atmosphere to degrease the green compact.

The sintering step comprises: a step 1 of nitriding the degreased green compact by heat treatment at a temperature of 1200 to 1500 ℃ in a nitrogen atmosphere; and a 2 nd step of sintering the obtained nitride body by heat treatment in, for example, a nitrogen atmosphere at 1600 to 1950 ℃ preferably 1600 to 1900 ℃. In the step 1, the temperature is preferably maintained at 1200 to 1500 ℃ (preferably 1300 to 1500 ℃) for a long time (for example, 1 hour or more) in order to completely nitridize silicon. In the present specification, the temperature maintenance means maintaining the temperature for a certain time. The temperature rise rate at the time of transition from step 1 to step 2 is, for example, 2 ℃/min or more, 2.5 ℃/min or more, or 5 ℃/min or more. The rate of temperature rise is, for example, 20 ℃/min or less, preferably 15 ℃/min or less.

As shown in examples described later, the nitriding can be promoted by adjusting the amount and/or particle diameter of the sintering aid and the type of the rare earth element. As a result, the temperature maintenance in the step 1 can be omitted. In addition, the temperature increase rate can be increased when the process shifts from step 1 to step 2. This can shorten the manufacturing time and improve the energy efficiency during manufacturing.

(method 2)

The method of 2 is a method for producing a silicon nitride sintered body containing a rare earth element and an aluminum element, and includes, for example: a mixing step of dry-mixing a raw material powder containing a silicon powder and a sintering aid, a molding step of molding the mixed raw material powder into a green compact, and a sintering step of sintering the green compact. The method 2 is different from the method 1 in that all steps of the PS-RBSN method are performed in a dry manner. After the sintering step, the silicon nitride sintered body may be polished, if necessary.

The mixing step is a step of mixing the raw material powder in a dry manner without using water or an organic solvent. In this step, it is preferable to mix the components without using a binder component. The particle size of the powder after mixing is not particularly limited, and D90 is preferably 10 μm or more and 100 μm or less, more preferably 10 μm or more and 50 μm or less, and further preferably 10 μm or more and 20 μm or less. The D50 is preferably 2 to 10 μm, more preferably 3 to 9 μm, and still more preferably 4 to 8 μm. By setting the D90 and/or D50 within the above range, a dense silicon nitride sintered body can be obtained while exhibiting good fluidity and moldability. D50 and D90 are a cumulative 50% diameter and a cumulative 90% diameter on a volume basis, respectively, and are obtained by laser diffraction scattering particle size distribution measurement or the like.

In the next molding step, the mixed powder is molded into a predetermined shape to obtain a green compact. The sintering step comprises: a1 st step of nitriding the obtained green compact by heat treatment at a temperature of 1200 to 1500 ℃ in a nitrogen atmosphere, for example; and a 2 nd step of sintering the sintered body by heat treatment at 1600 to 1950 ℃ preferably 1600 to 1900 ℃ in a nitrogen atmosphere, for example. From the viewpoint of improving the production efficiency, it is preferable that the temperature in the step 1 is not maintained at a temperature within the range of 1200 to 1500 ℃ for 1 hour or more. Specifically, it is preferable to increase the temperature from, for example, about 1100 ℃ to the sintering temperature in the above-mentioned step 2 at a predetermined temperature increase rate to nitride the silicon wafer. The temperature increase rate is, for example, 2 ℃/min or more, 2.5 ℃/min or more, or 5 ℃/min or more. The temperature increase rate is, for example, 20 ℃/min or less, preferably 15 ℃/min or less.

The method of 2 obtains the following effects as compared with the method of 1.

By performing all steps in a dry manner by the PS-RBSN method, for example, oxidation of silicon powder can be prevented when a water solvent is used, and environmental load due to an organic solvent such as ethanol can be reduced.

By producing a silicon nitride sintered body without using an organic binder in the PS-RBSN method, shrinkage due to sintering can be reduced, and the dimensional accuracy of the sintered body can be improved. In the case of method 1, since an organic binder or the like is used for granulation, a degreasing step is required thereafter, and since voids are generated after the organic binder is removed in the degreasing step, shrinkage due to sintering may increase accordingly.

In addition, by reducing the shrinkage, the polishing time in the subsequent polishing step can be shortened.

By producing a silicon nitride sintered body without using a binder component in the PS-RBSN method, the degreasing step can be omitted, and CO which may be generated by decomposition of the binder component in the degreasing step can be prevented ₂ The generation of the isothermal chamber effect gas can reduce the environmental load.

In general, conventional Si is used ₃ N ₄ The method of using the powder as a raw material to obtain a dense sintered body requires the use of fine Si ₃ N ₄ Powder (D50 is 1 μm or less). Since such fine powder is inferior in flowability and moldability, it is necessary to prepare a slurry of the raw material powder and the binder component with water, ethanol or the like, and spray-granulate and dry the slurry by spray drying or the like to obtain granules. However, in the PS-RBSN method, since Si powder is refined by fracture due to volume expansion in the nitriding step, it is not necessary to use Si powder for obtaining a dense sintered body ₃ N ₄ Fine powder is used as a raw material as powder. Since the raw material powder is not fine, the flowability and moldability required for obtaining a molded article can be ensured even if the raw material powder is not granulated powder.

In the production of the above silicon nitride sintered body comprising the above methods 1 and 2Among them, as the sintering aid used in the raw material powder, a sintering aid containing a rare earth element, an aluminum element, and a transition metal element is preferably used, and an oxide thereof is more preferably contained. As the sintering aid containing a rare earth element, Y is preferably contained ₂ O ₃ 、CeO ₂ 、Nd ₂ O ₃ And Eu ₂ O ₃ Any one of the above. The sintering aid containing a transition metal element preferably contains Cr ₂ O ₃ 、TiO ₂ MnO and Fe ₂ O ₃ More preferably, it contains Cr ₂ O ₃ 、TiO ₂ And Mn0, and further preferably contains Cr ₂ O ₃ 。

The raw material powder may contain silicon nitride powder and/or an organic binder in addition to the silicon powder and the sintering aid, and may contain a sintering aid containing elements other than rare earth elements, aluminum elements, and transition metal elements.

The content of the silicon powder contained in the raw material powder is preferably 45 wt% or more, more preferably 50 wt% or more, further preferably 55 wt% or more, and may be 60 wt% or more, usually 90 wt% or less, 85 wt% or less, or 80 wt% or less, based on the total weight of the silicon powder, the silicon nitride powder, and the sintering aid. The content of the silicon nitride powder contained in the raw material powder is usually 30 wt% or less, preferably 25 wt% or less, more preferably 20 wt% or less, and may be 15 wt% or less, based on the total weight, or may not contain the silicon nitride powder.

The content of the sintering aid containing a rare earth element (for example, an oxide of a rare earth element) contained in the raw material powder is 7 wt% or more, preferably 9 wt% or more, more preferably 9.5 wt% or more, and may be 10 wt% or more, based on the total weight. The content of the rare earth element is 17 wt% or less, may be 15 wt% or less, and may be 14.5 wt% or less. The content of the sintering aid (for example, alumina) containing an aluminum element in the raw material powder is 5 wt% or more, preferably 9 wt% or more, more preferably 9.5 wt% or more, and may be 10 wt% or more, based on the total weight. The content of the aluminum element is 17 wt% or less, may be 15 wt% or less, and may be 14.5 wt% or less. The content of the sintering aid containing a transition metal element (for example, an oxide of a transition metal element) contained in the raw material powder is usually preferably 0.1% by weight or more, more preferably 0.5% by weight or more, and usually 5% by weight or less, more preferably 3% by weight or less, based on the total weight. If the content of the sintering aid contained in the raw material powder is small, it is difficult to obtain a dense silicon nitride sintered body, and if the content of the sintering aid is large, mechanical properties such as fracture toughness of the silicon nitride sintered body tend to be lowered.

The average particle diameter of the silicon powder contained in the raw material powder may be, for example, 5 μm or less. The average particle diameter of the silicon nitride may be 0.5 μm or less, for example. The average particle size of the sintering aid varies depending on the type of the sintering aid, and is usually 10 μm or less, may be 7 μm or less, may be 5 μm or less, may be 3 μm or less, may be 2 μm or less, may be 1 μm or less, and may be 0.4 μm or less.

One embodiment of the above-described method 2 is a method for producing a silicon nitride sintered body containing a rare earth element and an aluminum element, which comprises a mixing step of dry-mixing a raw material powder containing a silicon powder and a sintering aid, a molding step of molding the mixed raw material powder into a green compact, and a sintering step of sintering the green compact, wherein the silicon powder is contained in an amount of 45 wt% or more based on the whole raw material powder.

Further, the above-described one aspect of the method 2 may have 1 or 2 or more of the following features (1) to (7).

(1) The mixing step is a step of mixing the raw material powder without using a binder component.

(2) The sintering step includes a step of raising the temperature at a rate of 15 ℃/min or less without holding a predetermined temperature for 1 hour or more while raising the temperature from a temperature in the range of 1000 to 1200 ℃ to the sintering temperature.

(3) The sintering temperature is in the range of 1600-1900 ℃.

(4) The sintering aid contains a rare earth oxide and alumina, the raw material powder contains the rare earth oxide in an amount of 9.5 wt% to 17 wt% with respect to the entire raw material powder, and the alumina in an amount of 9.5 wt% to 17 wt% with respect to the entire raw material powder.

(5) The rare earth oxide contains Y ₂ O ₃ 、CeO ₂ 、Nd ₂ O ₃ And Eu ₂ O ₃ 1 or more of them.

(6) The sintering aid contains a transition metal compound, and the raw material powder contains the transition metal compound in an amount of 0.1 to 5 wt% based on the total amount of the raw material powder.

(7) The transition metal element contains 1 or more kinds selected from Ti, cr and Mn.

For example, by adding 9.5 wt% to 17 wt% of a rare earth oxide and 9.5 wt% to 17 wt% of alumina as sintering aids to the raw material powder, the nitriding of silicon and the subsequent sintering can be promoted (item (4) above). Further, by adding 0.1 wt% or more and 5 wt% or less of a transition metal compound as a sintering aid, the nitridation of silicon can be promoted (item (6) above). By promoting the nitridation of silicon, the temperature is not required to be maintained at 1100 to 1450 ℃ for a long time in a nitrogen atmosphere, which is generally performed, and thus the method has excellent energy efficiency.

(use of silicon nitride sintered body)

The silicon nitride sintered body of the present embodiment can be used for bearing members, rolling rolls, compressor blades, gas turbine blades, engine parts, and the like, because the silicon nitride sintered body has excellent mechanical properties and thermal conductivity. The bearing member can be used, for example, as a bearing member such as a rolling bearing, a linear motion guide bearing, a ball screw, or a linear motion bearing, and can be suitably used, in particular, as a rolling element of a bearing.

The bearing of the present embodiment will be described with reference to fig. 1. Fig. 1 is a cross-sectional view of a deep groove ball bearing. In the rolling bearing 1, an inner ring 2 having an inner ring raceway surface 2a on an outer peripheral surface thereof and an outer ring 3 having an outer ring raceway surface 3a on an inner peripheral surface thereof are concentrically arranged, and a plurality of balls (rolling elements) 4 are arranged between the inner ring raceway surface 2a and the outer ring raceway surface 3 a. These balls 4 are formed of the above-described silicon nitride sintered body. The ball 4 is held by a retainer 5. Further, both axial end openings 8a and 8b of the inner and outer rings are sealed by a seal member 6, and a grease composition 7 is sealed at least around the ball 4. The grease composition 7 is interposed on the raceway surface of the ball 4 for lubrication.

(use of bearing)

The bearing of the present embodiment is not particularly limited in application, and functions as an insulating bearing by using a rolling element formed of a silicon nitride sintered body, and is therefore suitably used in a structure in which a current can flow in the bearing. For example, the present invention can be applied to a main motor, a general-purpose motor, a generator, and the like of a railway vehicle. In recent years, the present invention is also applicable to flying vehicles that have attracted attention as a means of moving instead of automobiles. The flying automobile is expected to solve various social problems, and is expected to be effectively used in various occasions such as regional movement, inter-regional movement, sightseeing, leisure, emergency medical treatment, disaster relief and the like.

As an automobile, a vertical Take-Off and Landing (VTOL) is attracting attention. Since the vertical take-off and landing machine can vertically lift and descend in the sky and the take-off and landing place, a runway is not required, and convenience is excellent. In particular, in recent years, CO is being turned on by society ₂ For example, an electric vertical take-off and landing (eVTOL) of a type flying with a battery and a motor has been mainly developed.

An electric vertical lift/lower machine having the bearing according to the present invention mounted thereon will be described with reference to fig. 2. The electric vertical lift 11 shown in fig. 2 is a multi-axis helicopter having a main body 12 located at the center of the machine body and 4 driving units 13 arranged in the front, rear, left, and right directions. The driving unit 13 is a device that generates lift force and propulsive force of the electric vertical lift 11, and the electric vertical lift 11 flies by the driving of the driving unit 13. In the electric vertical lifting/lowering machine 11, the number of the driving units 13 may be plural, and is not limited to 4.

The body 12 has a living space in which passengers (for example, about 1 to 2 passengers) can ride. In the living space, an operation system for determining a traveling direction, an altitude, and the like, meters for displaying an altitude, a speed, a flight position, and the like are installed. The 4 arms 12a extend from the main body 12, and a drive unit 13 is provided at the tip of each arm 12 a. In fig. 2, in order to protect the rotary blade 14, a circular ring portion that covers the rotation periphery of the rotary blade 14 is integrally provided on the arm 12 a. In addition, a lower part of the main body 12 is provided with a landing gear 12b for supporting the body during landing.

The drive unit 13 includes a rotary vane 14 and a motor 15 for rotating the rotary vane 14. In the driving portion 13, a pair of rotary blades 14 is provided on both sides in the axial direction with a motor 15 interposed therebetween. Each of the rotary blades 14 includes 2 blades extending radially outward.

The main body 12 is provided with a battery (not shown) and a control device (not shown). The control device is also referred to as a flight controller. The control of the electric vertical lifting/lowering machine 11 is performed by the control device, for example, as follows. The control device outputs a command for changing the rotation speed to the motor 15 for adjusting the lift force based on the difference between the current posture and the target posture. In response to this command, the amplifier provided in the motor 15 adjusts the amount of electric power transmitted from the battery to the motor 15, and changes the rotational speed of the motor 15 (and the rotary blade 14). Further, the rotational speed of the motor 15 is adjusted simultaneously for the plurality of motors 15, thereby determining the posture of the machine body.

Fig. 3 shows a partial cross-sectional view of the motor in the drive section. In fig. 3, the rotary blade is attached to one end (upper side in the drawing) of a rotary shaft 17 of the motor 15, and a rotor is attached to the other end (lower side in the drawing). The rotor is disposed opposite to the stator fixed to the housing and is rotatable with respect to the stator. The motor 15 may be configured as an outer rotor type brushless motor or an inner rotor type brushless motor.

In fig. 3, the motor 15 includes a housing (device housing) 16, a rotor (not shown), a stator (not shown), an amplifier (not shown), and 2 rolling bearings (deep groove ball bearings) 21 and 21. The casing 16 has an outer cylinder 16a and an inner cylinder 16b, and a coolant flow field 16c is provided therebetween. By flowing the cooling medium through the flow path 16c, an excessive temperature rise can be prevented. The rolling

bearings

21 and 21 rotatably support the rotary shaft 17 in the inner tube 16 b. In fig. 3, the ball 24 of the rolling bearing 21 is formed of the above silicon nitride sintered body. The rolling bearing 21 corresponds to the bearing of the present invention.

In the rolling bearing 21, the outer ring 23 has an outer diameter substantially the same as that of a fitting portion of the inner periphery of the housing, and is directly fitted to the housing 16 without passing through a bearing housing or the like. An inner ring spacer 18 and an outer ring spacer 19 are inserted between rolling

bearings

21 and 21, and a preload is applied. The outer ring spacer 19 is provided with nozzle members 20, and the

nozzle members

20, 20 are used to inject lubricating oil for cooling and lubricating the rolling

bearings

21, 21. The nozzle member 20 has a lubricating oil flow path therein for guiding oil gas supplied from an external lubricating oil supply device (not shown) to the bearing space.

In the electric vertical takeoff and landing machine, since the capacity of the motor is increased as compared with the unmanned aerial vehicle, the drive current is increased, and the voltage (shaft voltage) generated at the rotation shaft of the motor is increased. Meanwhile, although there is a concern that galvanic corrosion may occur, by applying a bearing including a rolling element formed of the above silicon nitride sintered body, galvanic corrosion due to energization can be appropriately prevented while having a good product life. As a result, the occurrence of an abnormality in the bearing is suppressed, and the electric vertical lift is useful for safe flight or the like. Further, by using the rolling elements made of a silicon nitride sintered body, the weight of the bearing can be reduced as compared with the rolling elements made of an iron-based material, and therefore, the bearing is particularly suitable for bearings of electric vertical hoists which are required to be reduced in weight.

The bearing structure of the driving unit is not limited to the structure of fig. 3. In fig. 3, the rotating shaft of the motor and the rotating shaft of the rotary blade are the same rotating shaft, but the rotating shaft of the motor and the rotating shaft of the rotary blade may be connected via a transmission mechanism. In this case, the rolling bearing that supports the rotary shaft in the drive unit may be a rolling bearing that supports the rotary shaft of the motor, or may be a rolling bearing that supports the rotary shaft of the rotary blade.

Examples

The present invention will be described more specifically below with reference to examples and comparative examples, but the present invention is not limited to these examples.

[ test example 1]

Raw material powders were prepared at the compounding ratios shown in table 2, to which 3 wt% of an organic binder was added, and mixed for 48 hours at a rotation speed of 200rpm using silicon nitride balls as a medium and ethanol as a solvent using a ball mill. And drying the mixed slurry by adopting a spray drying method, and granulating to obtain granulated powder. The specification of the material used for obtaining the granulated powder is shown in table 1.

[ Table 1]

Material	Manufacturing facility	Purity [% ]]	Average particle diameter [ mu m ]]
				Si	High purity chemistry	>99.9	～5
Si ₃ N ₄	High purity chemistry	>99.9	～0.5
				Al ₂ O ₃	High purity chemistry	>99.9	～1
Y ₂ O ₃	High purity chemistry	>99.99	～0.4
				CeO ₂	High purity chemistry	>99.0	～7
Nd ₂ O ₃	High purity chemistry	>99.9	～7
				Eu ₂ O ₃	High purity chemistry	>99.9	～5
Cr ₂ O ₃	High purity chemistry	>99.9	～3
				TiO ₂	High purity chemistry	>99.9	～2
MnO	High purity chemistry	>99.9	～5
				Fe ₂ O ₃	High purity chemistry	>99.9	～1

< examples 1 to 26, 30 and comparative examples 1 to 2>

Using the granulated powder obtained above, a spherical green compact having a diameter of 11mm was molded by a cold isostatic pressing method using a rubber mold. The green compact was degreased at 800 ℃ for 48 hours in a nitrogen atmosphere, and then heated to 1400 ℃ at a heating rate of 2.5 ℃/min, and the green compact was nitrided by being held at 1400 ℃ for 4 hours in a nitrogen atmosphere (pressure: 0.9 MPa). Then, the nitrided green compact is heated to a temperature of 1550 to 1950 ℃ at a heating rate of 2.5 to 20 ℃/min, and held at the sintering temperature for 4 hours in a nitrogen atmosphere (pressure: 0.9 MPa) to obtain a silicon nitride sintered body.

< examples 27 to 29>

Using the granulated powder obtained above, a spherical green compact having a diameter of 11mm was molded by a cold isostatic pressing method using a rubber mold. The green compact was degreased in a nitrogen atmosphere at 800 ℃ for 48 hours, and then heated at a heating rate of 20 ℃/min to 1800 ℃ and held at 1800 ℃ in a nitrogen atmosphere (pressure: 0.9 MPa) for 4 hours to obtain a silicon nitride sintered body. In examples 27 to 29, the step of nitriding at 1400 ℃ for 4 hours (temperature maintenance) was omitted.

The dimensions of the green compacts and the dimensions of the silicon nitride sintered bodies obtained in the examples and comparative examples were measured with a micrometer, and the shrinkage ratios were calculated from the following formulas. The shrinkage is shown in table 4 together with other measurement results.

Shrinkage ratio [% ] [ { (diameter of green compact) - (diameter of silicon nitride sintered compact) }/diameter of green compact ]. Times.100

The composition ratio of each oxide in the obtained silicon nitride sintered body was such that all silicon (metal silicon) contained in the raw material powder was nitrided, and the weight of silicon nitride was 1.67 times the weight of silicon, and values calculated from the composition ratio of the raw material powder were shown in table 3.

The obtained spherical silicon nitride sintered body was ball-polished to G5 in accordance with JIS B1563 to produce a spherical test piece of 3/8 inch (diameter: 9.525 mm).

[ Table 2]

[ Table 3]

< measurement of maximum diameter and area ratio of inclusions (I) and measurement of maximum diameter of voids >

The test pieces obtained in examples and comparative examples were cut into a cross section passing through the center thereof, and the cut surface was mirror-polished. The mirror-polished cut surface was photographed with "VHX5000" manufactured by Keyence corporation, the photographed image was analyzed with "winrofo" manufactured by mitsubishi corporation, and the maximum diameter of the inclusions (I) present in the surface layer portion, which is a region corresponding to a range within 2mm from the surface of the spherical test piece, and the maximum diameter of the pores were measured. The diameters of the inclusions (I) and the voids (the diameters of the inclusions (I) and the voids = √ the envelope areas of the inclusions (I) and the voids) are obtained as the square roots of the envelope areas of the inclusions (I) and the voids. A sample in which the inclusions (I) having a diameter of more than 50 μm were not present in the surface layer portion was evaluated as "A", and a sample in which the inclusions were present was evaluated as "B". In addition, a sample having no pores with a diameter of more than 50 μm in the surface layer portion was evaluated as "A", and a sample having pores with a diameter of more than 50 μm was evaluated as "B". The inclusions (I) and voids are measured with the inclusions (I) and voids existing entirely in the surface layer. Further, the ratio of the total cross-sectional area of the inclusions (I) to the total cross-sectional area of the surface layer portion (ratio of the total cross-sectional area of the inclusions (I) = envelope area of the inclusions (I) ÷ total cross-sectional area of the surface layer portion × 100) was calculated. The results are shown in Table 4.

< evaluation of fracture toughness >

The test pieces obtained in examples and comparative examples were cut into a cross section passing through the center thereof, the cut surface was mirror-polished, and the value of fracture toughness was measured according to JIS R1607.

< measurement of crush Strength >

The test pieces obtained in examples and comparative examples were used to perform a 2-ball crushing test. The crush test was according to JIS B1501.

< Rolling fatigue test >

The test pieces obtained in examples and comparative examples were used as the bearing outer ring, the bearing inner ring and the cage, and a rolling fatigue test was carried out at a rotation speed of 3000rpm, a load of 1.5GPa and a test time of 168 hours by using "6206" manufactured by NTN K.K., to evaluate the product life. As the lubricating oil, use was made of a non-additive turbine oil "VG56" manufactured by JXTG energy Co. The sample of the test piece that did not peel off during the test time was evaluated as "a", and the sample that peeled off was evaluated as "b". The results are shown in Table 4.

[ Table 4]

< analysis of the inclusions (I) >

The cut surface of the test piece obtained in example 6 was measured for the type and content of the element of the inclusion (I) contained in the surface layer portion by EDX analysis using a scanning electron microscope (S300, manufactured by hitachi corporation). The inclusion (I) contains chromium silicide, and the elements and the contents thereof contained in the inclusion (I) are as follows: chromium (Cr) 56 wt% and silicon (Si) 44 wt%.

[ test example 2]

In test example 2, granulated powder was obtained by dry mixing except for example 30. First, the raw material powders shown in table 1 were prepared at the blending ratios shown in table 2.

< examples 1 to 29 and comparative examples 1 and 2>

Silicon nitride balls were used as a medium, and dry-mixed for 48 hours at a rotation speed of 200rpm using a ball mill. Using the obtained mixed powder, a spherical green compact having a diameter of 11mm was molded by a cold isostatic pressing method using a rubber mold. The green compact was heated from room temperature to a temperature of 1550 to 1950 ℃ at a heating rate of 2.5 to 20 ℃/min shown in Table 2, and held at the sintering temperature in a nitrogen atmosphere (pressure: 0.9 MPa) for 4 hours to obtain a silicon nitride sintered body.

< example 30>

The raw material powder was mixed with 3 wt% of an organic binder added to the entire raw material powder, using silicon nitride balls as a medium and ethanol as a solvent, at a rotation speed of 200rpm for 48 hours by a ball mill. And spraying and drying the mixed slurry by adopting a spray drying method to obtain granulation powder. Using the granulated powder thus obtained, a spherical green compact having a diameter of 11mm was molded by a cold isostatic pressing method using a rubber mold. The green compact was degreased in a nitrogen atmosphere at 800 ℃ for 48 hours, and then heated at a heating rate of 2.5 ℃/min to 1800 ℃ and held at 1800 ℃ in a nitrogen atmosphere (pressure: 0.9 MPa) for 4 hours to obtain a silicon nitride sintered body.

The dimensions of the green compacts and the dimensions of the silicon nitride sintered bodies obtained in examples and comparative examples were measured with a micrometer, and the shrinkage ratios were calculated in the same manner as in test example 1. The results are shown in Table 5.

The obtained spherical silicon nitride sintered body was ball-polished in accordance with JIS B1563 to G5 to prepare a spherical test piece of 3/8 inch (diameter: 9.525 mm).

Using the test pieces obtained in the obtained examples and comparative examples, the maximum diameter and area ratio of the inclusions (I), the maximum diameter of the pores, the fracture toughness evaluation, the crush strength measurement, and the rolling fatigue test were performed in the same manner as in test example 1. The results are shown in Table 5.

[ Table 5]

Next, a comparative study was conducted on a test piece produced by wet granulation and a test piece produced by dry mixing. As the test piece, example 27 (see tables 4 and 5) in which good results were obtained was used.

< measurement of maximum diameter of void >

Using each test piece of example 27, the maximum diameter of the pores present in the surface layer portion was measured in the same manner as in test example 1. From the results of tables 4 and 5, in each test piece, no pores having a diameter of more than 50 μm were present in the surface layer portion. This time, the presence or absence of pores having a diameter of 10 μm or more was further evaluated. The results are shown in Table 6.

< Rolling fatigue test >

Using the test pieces of example 27, rolling fatigue tests were performed under higher load conditions than in test example 1. The same conditions were used except that the test conditions of test example 1 were changed to a load of 3.5GPa and a test time of 630 hours. The test piece was evaluated for peeling within the test time. The results are shown in Table 6.

[ Table 6]

As shown in Table 6, in example 27 (wet type), pores having a diameter of 10 μm or more and less than 50 μm were present, whereas in example 27 (dry type), pores having a diameter of 10 μm or more were not present. In example 27 (dry type), no peeling occurred in the rolling fatigue test under the high load condition. In the case of wet granulation using granulation with an organic binder or the like, it is considered that the granulated powder is hard and is hard to be sufficiently crushed by pressurization, and a gap is likely to remain on the surface of the granulated powder bonded to the molded body, as compared with dry mixing. This may cause defects in the sintered body depending on the use mode. In the rolling fatigue test of example 27 (wet type), it is considered that the sintered body used as the rolling element under a high surface pressure causes the defects along the joint surface to be threshed, resulting in a reduction in the life.

The embodiments and examples disclosed herein are illustrative in all respects and should not be considered as limiting. The scope of the present invention is defined by the claims, not by the embodiments described above, and is intended to include all modifications within the scope and meaning equivalent to the claims.

Industrial applicability

The silicon nitride sintered body of the present invention can be suitably used for rolling elements of bearings such as rolling bearings, linear motion guide bearings, ball screws, and linear motion bearings.

Description of the reference numerals

1. Rolling bearing

2. Inner ring

3. Outer ring

4. Rolling body

5. Retainer

6. Sealing member

7. Lubricating grease

8a, 8b openings

11. Electric vertical take-off and landing machine

12. Main body part

13. Driving part

14. Rotating blade

15. Motor with a stator having a stator core

16. Shell body

17. Rotating shaft

18. Inner ring spacer ring

19. Outer ring spacer ring

20. Nozzle component

21. Rolling bearing

22. Inner ring

23. Outer ring

24. Ball with ball-shaped section

Claims

1. A silicon nitride sintered body containing a rare earth element and an aluminum element,

the content of the rare earth element is 6 to 13 wt% in terms of oxide based on the total weight of the silicon nitride sintered body,

the content of the aluminum element is 6 to 13 wt% in terms of oxide with respect to the total weight of the silicon nitride sintered body.

2. The silicon nitride sintered body according to claim 1, wherein the rare earth element contains 1 or more selected from Y, ce, nd, and Eu.

3. The silicon nitride sintered body according to claim 1, wherein the rare earth element contains Ce.

4. The silicon nitride sintered body according to claim 1, wherein an inclusion (I) is present in a surface layer portion which is a region within 2mm from the surface of the silicon nitride sintered body, and a ratio of a total cross-sectional area of the inclusion (I) to a total cross-sectional area of the surface layer portion is 0.05% or more.

5. The silicon nitride sintered body according to claim 4, wherein the inclusion (I) contains an inclusion (It) containing a transition metal element.

6. Silicon nitride sintered body according to claim 5, characterized in that the inclusion (It) is a silicide of a transition metal element.

7. The silicon nitride sintered body as claimed in claim 5, wherein the transition metal element contains 1 or more selected from Ti, cr and Mn.

8. The silicon nitride sintered body as claimed in claim 5, wherein the transition metal element contains Cr.

9. The silicon nitride sintered body according to claim 4, wherein the maximum diameter of the inclusion (I) is 50 μm or less.

10. The silicon nitride sintered body according to claim 1, wherein pores having a maximum diameter of 50 μm or less are present in a surface layer portion which is a region within 2mm from the surface of the silicon nitride sintered body.

11. A rolling element, characterized in that the silicon nitride sintered body according to claim 1 is used.

12. Bearing, characterized in that a rolling element according to claim 11 is used.

13. The bearing according to claim 12, wherein the bearing is mounted on a drive unit including a plurality of rotor blades and a motor for rotating the rotor blades, an electric vertical lift machine flying by rotation of the rotor blades, and a bearing for supporting a rotating shaft in the drive unit.